1. Field of the Invention
This invention relates to a process for removal of solids from a liquid containing suspended solids by flowing the liquid through a filter bed of discontinuous polyurethane particles.
2. Description of the Prior Art
In the field of liquid and wastewater treatment, filtration has long been a major method of removing suspended solids from liquid streams. In wastewater treatment applications in particular, the presence of suspended solid materials is frequently a major process problem and filtration has commonly been employed as a means of reducing and/or removing suspended solids from streams such as municipal sewage and wastewater intended for recycle use. In such applications, downflow and upflow sand filters and dual or mixed media filters have been widely employed and have in general been shown to be cost effective and efficient in use. Nonetheless, work by practitioners in the field has shown that sand and mixed media filters are in general effective in removing suspended solids, but only under limited solids loading conditions. In general, solids concentration of the liquid stream entering the filter must be below about 100-200 milligrams/liter. At suspended solids concentration values above this level, the filtration bed is susceptible to clogging and high pressure drop across the bed.
In recent years, the operation of filtration has been somewhat improved in the foregoing applications by the use of polyurethane as a filtration medium. In general practice, polyurethane foam is cut up into small pieces and placed into a retaining vessel to form the filtration bed. Wastewater or other liquid containing suspended solids is then flowed through the bed with resulting deposition of suspended solids on the polyurethane particles. The suspended solids thus remain on the polyurethane particles and the liquid from which the suspended solids has been deposited subsequently passes through the bed and is discharged as liquid depleted in suspended solid contaminants.
In general, the use of polyurethane foam as a filtration medium provides numerous operating advantages over beds employing sand or conventional mixed media filtration materials, including higher solids capacity, lower pressure drop head losses, higher resistance to clogging and removal of numerous soluble organic contaminants from the liquid stream.
Despite its numerous operating advantages, however, regeneration of the polyurethane foam which has become loaded with suspended solids in the filtration operation has continued to pose a severe operating problem. Various methods have been contemplated for regeneration of the polyurethane foam filtration bed which has become at least partially loaded with suspended solids during the filtration step, including back-washing with a solids-depleted liquid stream, such as is commonly employed in conventional sand and mixed media filtration systems. The use of back washing in a polyurethane foam filtration bed poses a specific problem due to the low density of the polyurethane foam filtration medium. Since polyurethane has a true density which may be as low as 1-2 pound/foot.sup.3, it is generally desirable in normal operation to flow liquid containing suspended solids downwardly through the bed of polyurethane foam material. Back washing of such a filtration bed, which involves counter-currently flowing a stream of clean liquid upwardly through the filtration bed, in general requires such high flow velocities through the bed as to wash particles of polyurethane foam out of the bed or else causes such disruption of the bed as to leave large void spaces therein which contribute to bypassing and other disadvantageous performance behavior in the subsequent filtration step. If, on the other hand, the bed of polyurethane foam is regenerated in the same manner as the normal filtration step, by flowing a stream of solids-depleted liquid downwardly through the solids-loaded bed, it is generally difficult to obtain sufficient removal of suspended solids from the polyurethane particles to satisfactorily renew the bed for the subsequent filtration step.
Faced with the foregoing difficulties in the application of conventional back washing to the polyurethane foam filtration bed, and in view of the extremely low density of the polyurethane foam filtration medium relative to conventional media, the prior art has proposed various means for compression regeneration of the polyurethane foam in the filtration bed. Such compression regeneration generally involves the use of mechanical means for "squeezing" the particles of polyurethane foam, thereby "wringing out" the contained water along with the deposited solids from the particles of polyurethane foam. Although such method of regeneration is easily employed on a laboratory or bench-scale filtration unit, the mechanical complexity and attendant capital expense of means for squeezing the foam particles in commercial scale polyurethane foam filtration beds has severely limited the utility of polyurethane foam filtration beds in practice.
Accordingly, it is an object of the present invention to provide an improved process for removing deposited suspended solids from polyurethane particles in a filtration bed of such particles.
It is another object of the invention to provide such a process which requires only a very low volume of regeneration liquid.
Other objects and advantages of the present invention will be apparent from the ensuing disclosure and appended claims.